Children conceived by in vitro fertilization (IVF) are at greater risk of certain health problems, the first comprehensive analysis of medical data has found. However, the cause remains unknown.

Around 1% of babies born in the United States start off life in the laboratory, and there has been growing concern that IVF might subtly affect aspects of children's health ranging from birth defects to cancer. But so far, the results of studies examining IVF children have been conflicting or incomplete.

Now a panel of experts has systematically analysed all the available medical literature and concluded that there is at least some increased risk from conceiving in a fertility clinic. The panel reported their preliminary results, based on 169 published studies, at the annual meeting of the American Society for Reproductive Medicine in Philadelphia on Tuesday.

Babies born after IVF are at least twice as likely to die at or soon after birth, or to be born prematurely or with a clinically low birth weight, the panel found. Premature and low-birth-weight babies are thought to suffer more health and developmental problems later in life. The evidence also suggested that IVF children are at greater risk of some rare genetic abnormalities.

“What we found should be very comforting.”

Panel head Kathy HudsonJohns Hopkins University in Baltimore

The panel found that these increased risks apply only to babies born singly; twins born after IVF were at no greater health risk than twins born conventionally. However, doctors know that bearing twins or triplets is itself linked to a range of long-term health problems, and that IVF boosts the risk of multiple births tenfold.

Parental role?

In some respects, however, the group's findings are reassuring, says panel head Kathy Hudson of Johns Hopkins University in Baltimore. The evidence does not show any increase in major birth malformations, cancer or psychological development, in contrast to some recent scares. "What we have found should be very comforting," she says.

Panel members say they can't be sure exactly what causes the problems in IVF children. One possibility is that the technique of fertilizing and growing young embryos in the lab somehow disrupts their normal development, resulting in problems later on.

“I think these women are different.”

Panel member Marcelle CedarsUniversity of California, San Francisco

But it is also possible that infertile couples themselves are the source of the problem, perhaps because they pass on detrimental genetic sequences to their babies. Alternatively, infertile mothers may be more likely to have their own health difficulties during pregnancy which then affect their children. "I think these women are different," says panel member and reproduction expert Marcelle Cedars of the University of California, San Francisco.

Hudson says the panel will next come up with a set of recommendations for how to work out the cause of these health problems and, potentially, how to solve them.

For example, larger, longer and more detailed studies of children born through IVF might reveal particular groups of parents who are at greater risk of health problems, and who could be given additional testing or support. Alternatively, detailed laboratory studies might show that problems can be avoided if embryos are nurtured in one growth solution rather than another.

The panel members acknowledge that parents will probably choose to pursue IVF regardless of the study's conclusions, because they are willing to accept the moderate health risks in exchange for the chance to bear children. But "you have to adequately inform patients of what those risks are", Cedars says.